1. Field of the Invention
The present invention relates to resonance adjustment of freely vibrating bodies. In particular, this invention relates to new and improved apparatus for automatic tuning of stringed musical instruments.
For purposes of the following discussion, the terms "pitch" and "tune" will be used interchangeably and will refer to the fundamental frequency of vibration of an instrument's strings.
2. Description of the Related Art
All stringed musical instruments require tuning due to changes in physical conditions or changes in the characteristics of the materials from which the instruments are made. Many stringed instruments, such as guitars and violins, drift out of tune quite rapidly and musicians often need to make tuning adjustments during the course of a performance.
Stringed instruments are presently manually tuned. The musician adjusts each string's tension (and hence its pitch) by mechanical means, such as worm gears. As there is no direct method for determining when a string is in tune, musicians must either tune their instruments "by ear" or use tuning aids.
Tuning "by ear" means that the musician uses his or her judgment to determine if a note is in tune. It is a difficult process that requires the ability to discern slight variations in pitch.
Tuning aids provide musicians with either an audio or visual reference in order to determine which way the string's pitch needs to be adjusted (higher or lower). Audio tuning aids, such as tuning forks, while considerably easier than tuning "by ear," still require the musician to judge when the string is in tune.
Visual tuning aids, such as those disclosed in U.S. Pat. Nos. 4,023462 (Denov et al), 4,088,052 (Hedrick) and 4,196,652 (Raskin), utilize electronics to measure the frequency of each string and compare it with an electronically generated reference frequency. A visual display is produced, indicating the magnitude and direction of the tuning error. The musician then adjusts each string to eliminate the error. Visual tuning aids allow individuals with very poor tone recognition skills to tune their instruments, but the actual tuning is still performed manually.
There are some tuning devices and tuning apparatus which are automatic in nature, such as those listed in Table I, below.
TABLE I ______________________________________ Patentee U.S. Pat. No. Issue Date ______________________________________ Scholz 4,375,180 March 1, 1983 Scholz 4,426,907 January 24, 1984 Minnick 4,584,923 April 29, 1986 Skinn et al 4,803,908 February 14, 1989 ______________________________________
Nonetheless, these automatic tuning devices and apparatus rely on methods which are inferior to the method of this invention.
Both Scholz patents rely on tension sensing means for determining frequency. As there is no linear correlation between frequency and tension of a string, this method is inaccurate.
Neither Minnick nor Skinn et al (hereinafter "Skinn") use tension sensing means to determine frequency; both utilize electronic means for comparing signals produced against reference signals. In both cases, a difference between signal produced and reference signal will activate motors which will then adjust string tension.
There are several disadvantages to this type of method. One significant disadvantage is the relative bulk of such a device or apparatus when attached to an instrument. The size of such an apparatus or device would make it difficult to incorporate into a musical instrument, especially the smaller ones (e.g. violins).
Another disadvantage to the methods of Minnick and Skinn is the use of motors to change string tensions. Since the comparison of the output and reference signals is electronic, the accuracy of this method is limited by the mechanical means of adjusting string tension.
Both Minnick and Skinn contemplate the use of motor-driven gears to effectuate actual adjustment of string tension. There is an inherent stability and control problem in the use of gears due to the existence of "backlash" (i.e. the play between two meshing gears). Although this "backlash" can be minimized, it cannot be eliminated altogether. In the course of ordinary use, gears and motors become worn and periodically need replacement. Furthermore, motor driven gears may to slow in response for effective tuning due to the slow response of gear reductions, signal conversions, inertia and inductive phase lag.
Another problem is the feedback associated with the gear train and electric motors. Hysteresis, due to gear backlash, and the phase lag inherent with inductive motors is likely to result in "hunting", where string tension adjustments overshoot the proper level and the system oscillates. None of the aforementioned patent publications address this problem.
The heat generated by servo motors and especially stepper motors, shown by Skinn, is a significant problem. Thermal drift is probably the primary cause of instruments going out of tune. Placing such heat sources within the instrument would make short term tuning drifts inevitable. Thermal cycling is also detrimental to the instrument itself.
The disadvantages pointed out in the prior art referenced above are overcome in this present invention by the elimination of gears and motors and the use of a piezoelectric element to effectuate actual adjustment of string tension.